Images of the interiors of bodies may be acquired using various types of tomographic techniques, which involve recording and measuring radiation from tissues and processing acquired data into images.
One of these tomographic techniques is positron emission tomography (PET), which involves determining spatial distribution of a selected substance throughout the body and facilitates detection of changes in the concentration of that substance over time, thus allowing to determine the metabolic rates in tissue cells.
The selected substance is a radiopharmaceutical administered to the examined object (e.g. a patient) before the PET scan. The radiopharmaceutical, also referred to as an isotopic tracer, is a chemical substance having at least one atom replaced by a radioactive isotope, e.g. 11C, 15O, 13N, 18F, selected so that it undergoes radioactive decay including the emission of a positron (antielectron). The positron is emitted from the atom nucleus and penetrates into the object's tissue, where it is annihilated in reaction with an electron present within the object's body.
The phenomenon of positron and electron annihilation, constituting the principle of PET imaging, consists in converting the masses of both particles into energy emitted as annihilation photons, each having the energy of 511 keV. A single annihilation event usually leads to formation of two photons that diverge in opposite directions at the angle of 180° in accordance with the law of conservation of the momentum within the electron-positron pair's rest frame, with the straight line of photon emission being referred to as the line of response (LOR). The stream of photons generated in the above process is referred to as gamma radiation and each photon is referred to as gamma quantum to highlight the nuclear origin of this radiation. The gamma quanta are capable of penetrating matter, including tissues of living organisms, facilitating their detection at certain distance from object's body. The process of annihilation of the positron-electron pair usually occurs at a distance of several millimetres from the place of the radioactive decay of the isotopic tracer. This distance constitutes a natural limitation of the spatial resolution of PET images to a few millimetres.
A PET scanner comprises detection devices used to detect gamma radiation as well as electronic hardware and software allowing to determine the position of the positron-electron pair annihilation event on the basis of the position and time of detection of a particular pair of the gamma quanta. The radiation detectors are usually arranged in layers forming a ring around object's body and are mainly made of an inorganic scintillation material. A gamma quantum enters the scintillator, which absorbs its energy to re-emit it in the form of light (a stream of photons). The mechanism of gamma quantum energy absorption within the scintillator may be of dual nature, occurring either by means of the Compton's effect or by means of the photoelectric phenomenon, with only the photoelectric phenomenon being taken into account in calculations carried out by current PET scanners. Thus, it is assumed that the number of photons generated in the scintillator material is proportional to the energy of gamma quanta deposited within the scintillator.
When two annihilation gamma quanta are detected by a pair of detectors at a time interval not larger than several nanoseconds, i.e. in coincidence, the position of annihilation position along the line of response may be determined, i.e. along the line connecting the detector centres or the positions within the scintillator strips where the energy of the gamma quanta was deposited. The coordinates of annihilation place are obtained from the difference in times of arrival of two gamma quanta to the detectors located at both ends of the LOR. In the prior art literature, this technique is referred to as the time of flight (TOF) technique, and the PET scanners utilizing time measurements are referred to as TOF-PET scanners. This technique requires that the scintillator has time resolution of a few hundred picoseconds.
CT (Computed Tomography) is one of the transmission methods used for imaging. This technique involves a measurement of X-ray radiation that penetrates through the object's body. Examination with the use of computer tomography involves multiple irradiation of the object with a suitably formed, i.e. collimated x-ray beam, and CT detectors measure the final intensity of the radiation beam, which, after penetration through the object's body, is weakened to various degrees—depending on the type of tissue through which radiation penetrates. The obtained signals provide the object's anatomical image on the basis of information concerning the distribution of electron density in tissues.
Superimposing of a functional image (PET) over an anatomical image (CT) considerably increases the capabilities of imaging techniques: a PET image enables precise positioning of metabolic changes in individual organs and the determination of the degree of these changes, whereas the obtainment of a CT image at the same time allows a precise allocation of these changes to respective organs. Obtained hybrid PET/CT images may be useful in scientific research on physiological processes and in testing the action of new medicines, where it is especially important to precisely assign to respective tissues metabolic changes of tested radiopharmaceuticals, during imaging.
In currently used PET/CT tomographs, the PET detector ring is spatially separated from the CT detector ring by ca. 60 cm. Therefore, imaging: PET and CT are actually carried out at various positions of the examined object and at different times. During the first examination stage, a CT scan is done in such a way that the object is continuously shifted along the tomograph, and then a PET tomograph examination is carried out, in which, in order to generate an image larger that the width of the PET tomograph detection ring, the object is shifted between individual imaging events in increments, ca. at ⅔ of the tomograph detection field width. Therefore, body imaging within an area larger than the longitudinal PET field of view requires that the object should be put in motion and stopped between individual imaging events. This procedure involves a threat that image distortions, so-called artefacts, may occur, especially in abdominal cavity organs, which may move between individual scanning events due to accelerations to which the object is subjected during shifting. Moreover, the superimposing of PET and CT images, taken in different times, requires that additional corrections should be introduced due to the weakening activity of the radiopharmaceutical and metabolic processes; what also needs to be remembered is that each of these corrections is additionally exposed to systemic errors that occur when the images are superimposed.
Patent literature lists solution methods with respect to the described difficulties, and tomographs enabling PET and CT scanning.
An American patent description, U.S. Pat. No. 7,170,971, presents a tomograph in which scanning with CT and SPECT detectors can be done at the same time (i.e. detectors used in single photon emission computed tomography). Detectors are placed on the tomograph arm that has two angular degrees of freedom, and additionally, also three translation degrees of freedom, within a specific range. The solution allows a sequential imaging, applying various methods. Also, imaging with the use of two different methods is possible at the same time; what must be borne in mind is that due to a small solid angle that can be covered by detectors in the described configuration, this method does not enable a simultaneous imaging of the entire body or larger body parts. The described device is optimised so that it can carry out imaging of individual organs, such as the heart, while the CT tomograph used in this solution is a second-generation tomograph combined with a conical x-ray radiation beam. Such a solution would be impractical in hybrid PET/CT tomographs.
A solution is known from an American patent application US20020090050, which enables the use of the same detectors to carry out PET and CT tomography within a specific area on the object's body, which allows a reduction of size and mass of the tomograph arm. Electronic circuits are connected to PET and CT detectors, processing discrete signals as well as integrating systems. PET and CT imaging is done sequentially: the tomograph arm rotates and a CT image is collected; then, the detector configuration is changed, the tomograph arm rotates and a PET image is collected. During scanning, the object is shifted against the detectors. The tomograph works in three data collection modes: discrete for PET, and integrating for CT, and it can collect discrete signals during CT acquisition. The configuration of detectors in the presented solution does not, however, permit the scanning of a full perimeter around the object. Moreover, the detector configuration must be changed in such a manner that in one setting, two detector blocks are shifted to one side for CT imaging, whereas in the other, detector blocks are positioned opposite each other, for PET imaging. On the basis of the presented solution, a lo detector can be built, which would cover the entire perimeter around the object, with a tube generating x-ray radiation, rotating outside the detectors' axis. However, the solution with the tube rotating outside the detector will not allow the obtainment of good CT image resolutions in a full object's volume, as far as a tomograph with a large longitudinal field of view is concerned.
In article “A Modular VME Or IBM PC Based Data Acquisition System For Multi-Modality PET/CT Scanners Of Different Sizes And Detector Types” (D. B. Crosetto et al., The Internet Journal of Medical Technology 2003 Vol. 1. No. 1), a device was described, which enabled simultaneous PET and CT imaging of the entire object's body. This solution contained detectors presented in US20020090050 patent application. The solution eliminates the formation of potential artefacts caused by the object movement between subsequent imaging sessions. According to the solution presented, each detection module is built of three types of crystals: CsI(TI), LSO, GSO. Out of the discussed crystals, LSO has the best time properties, enabling the use of information concerning differences during the registration of annihilation quanta: TOF-PET. However, the signal decay time, even for LSO crystal, being 40 ns (nanoseconds), is such large that allow data collection in CT tomography, in a signal counting mode at a maximum of 107 signals per converter. Moreover, the operation of signals in an integration mode, as described in the CT detector solution, is distorted by fluorescent effects.
Currently, intensive research is carried out concerning the design of such CT and PET detectors that would successfully satisfy stringent requirements with respect to PET and CT tomographs. In a doctoral thesis, entitled: “Combined detector in positron and x-ray tomography” (A. T. Nassalski, The Andrzej Soltan Institute for Nuclear Studies, Świerk 2010), results of research on PET and CT tomographs are presented.
The currently used PET technology is costly, mostly due to the prices of scintillators and electronics; also, it must be remembered that the cost of a conventional PET detector and electronics increases in proportion with the length of the longitudinal field of view in PET detectors. Therefore, one of the factors limiting mass production of hybrid PET/CT tomographs is high production cost of the PET tomographs with a large longitudinal field of view.
Patent application WO2011008119 describes an invention concerning the strip device and the method used in the determination of position and time of gamma quanta reaction, and the application of this device in PET tomography. The TOF-PET tomograph, described in the application, allows simultaneous imaging of whole object, while the material used to register gamma quanta is polymers doped with elements of high atomic numbers. The device described in this application reduces PET costs; however, the application does not describe a method of simultaneous PET & CT imaging with the use of polymer strip scintillators.
A Japanese patent application JP2004350942 discloses a tomographic device and a radiographic testing device simplifying a procedure in simultaneous imaging of a plurality of modalities and advancing the correction processing. An X-ray which is emitted from a circumferentially moving X-ray source and transmitted through a subject is detected using multiple radiation detectors disposed annularly and a plurality of pairs of gamma-ray emitted from the subject are detected. A computer selects an interest area inside the subject from an X-ray CT image and performs an image processing of function information on the subject using sectional or volume information on the selected interest area. The PET and CT detectors are arranged in a single detection layer. The detectors are made of an inorganic material (CdTe) and have a cubical shape.
It would be expedient to provide an imaging device with the use of economical polymer scintillators, which would enable simultaneous registration of gamma quanta and x-ray radiation quanta with a broad field of view, enabling the elimination of artefacts that could distort the image due to the movement of the object, and systematic errors formed during superimposure of images made at various positions and times. This will allow effective, simultaneous functional and anatomical imaging.